The present embodiments relate to a local coil system for a magnetic resonance system for capturing MR signals.
A magnetic resonance system includes a tomograph, in which a patient on a couch is positioned in a cylindrical measurement chamber. A strong magnetic field that includes a gradient due to the activation of a number of gradient coils is established in the measurement chamber. The nuclear spin of atoms is aligned by the magnetic field. A transmitting antenna arrangement (e.g., a whole-body transmitting antenna arrangement such as a birdcage antenna) is located in the tomograph for the purpose of emitting the magnetic resonance high-frequency pulses in order to excite the atoms.
With regard to the reception of magnetic resonance signals (MR signals), local coils may be used in the context of a magnetic resonance examination in order to receive the pulses during the relaxation of the nuclear spin. Different materials have different relaxation characteristics, such that conclusions may be drawn in relation to the interior of the patient body on the basis of the relaxation characteristics. The local coils may be combined to form modules (subsequently referred to as “local coil systems”) and feature receiving antenna elements in each case (e.g., conductor loops). The received MR signals may be preamplified in the local coil and routed out of the central region of the magnetic resonance installation via cable and supplied to a screened receiver of an MR signal processing device, where the received data is digitized and processed further. In many examinations, a plurality of such local coils is already arranged around the patient in order to cover large regions of the body of the patient.
The functioning of magnetic resonance systems is known and is described in Imaging Systems for Medical Diagnostics, Arnulf Oppelt, Publicis Corporate Publishing, ISBN 3-89578-226-2, for example.
The local coils may be arranged in a local coil blanket that is placed over or under the patient body. Other specially shaped local coil systems such as, for example, head coils may be used. The signals may be carried by cables from the local coil systems to an evaluation device of the magnetic resonance system. The cables are undesirable, since the cables are not easily routed from the patient couch to the evaluation device. The cables are perceived as a nuisance by the staff, and the patient couch (including the patient and the local coil blanket) is mobile. Therefore, the cables are to be routed such that the cables are slack. Therefore, a wireless transfer of the data from the local coil system to the MR signal processing device is desirable. It may be more advantageous to digitize the relevant MR signals already in the local coil system and to transfer the MR signals in digital form.
In the local coil system, energy is used for the purpose of preprocessing the MR signals (e.g., for preamplification, digitization and encoding). Since the aim of the wireless transfer of the data from the local coil system is that a cable connection of the local coil system may be dispensed with completely, the local coil system either includes an adequate energy store, or the energy is also transferred wirelessly to the local coil system.
Microwaves may be used for energy transfer. However, microwave radiation is undesirable, as microwave radiation increases the physiological HF exposure of the patient.
A further option is to divert energy from the magnetic resonance transmission field by attaching a high-frequency rectifier and a buffer (e.g., a capacitor or an accumulator) to an off-resonance circuit of the receiving coils. This has the disadvantage that a current flows at the magnetic resonance frequency when power is diverted into the receiving coils, thereby distorting the field that excites the spins.